The present invention relates to (a) non-aqueous-base wellbore muds or fluids (e.g., drilling fluids, completion fluids, workover fluids, packer fluids, spotting fluids, coring fluids, perforating fluids, gravel packing fluids, and hydraulic fracturing fluids); (b) methods for using such wellbore fluids (e.g., drilling, completion, workover, packing, spotting, coring, perforating, gravel packing, and hydraulic fracturing operations); and (c) systems (for drilling a well in or for producing a natural resource from or for hydraulically fracturing a subterranean formation) having a drilling fluid or other wellbore fluid present therein. As used in the specification and claims, the term "non-aqueous-base" as used to modify the terms "wellbore mud", "wellbore fluid", and any specific type of wellbore mud or fluid such as drilling fluids, completion fluids, packer fluids, etc. means that a non-aqueous fluid is the continuous phase of such mud or fluid.
While non-aqueous-base wellbore drilling fluids are generally superior in comparison to water-base wellbore fluids and have undergone many changes since their introduction, there is still a need for further improvement. For example, the development of non-aqueous-base drilling fluids began as early as 1920 when engineers recognized that exposure of certain formations to filtrate from water-base fluids resulted in a loss of productive capacity. Non-aqueous-base drilling fluids have many advantages over water-base drilling fluids including, but not limited to, (1) the ability to drill a gauge hole, (2) superior shale inhibition, (3) high lubricity, (4) high temperature stability, (5) low chemical reactivity (e.g., non-reactive with chemicals in a subterranean formation), and (6) high rates of drill penetration due to synergism with polycrystalline diamond compact drilling bits.
However, since their inception, non-aqueous-base drilling fluids have developed into complex, expensive systems that generally contain (1) a petroleum derived--or synthetic fluid, (2) water, (3) a primary emulsifier, (4) a secondary emulsifier, (5) a wetting agent, (6) a viscosifying agent (e.g., an oil-wettable organophilic clay and/or a polymer), (7) lime, (8) a weighting agent, and, quite frequently, (9) a shale inhibiting salt (e.g., calcium chloride and/or sodium chloride), (10) calcium carbonate, (11) a thinner, (12) a fluid loss control agent (e.g., asphaltenes, gilsonite, oil-wettable lignite (such as amine-treated lignites), and (13) a polar activator, and, when needed, (14) a lost circulation material (e.g., granular materials (such as walnut and pecan shells), flaked materials (such as cellophane and polyethylene flakes), and fibrous materials (such as wood fibers, plant fibers, and synthetic fibers)).
Despite the numerous advantages of non-aqueous-base drilling fluids over water-base drilling fluids, the complexity and high cost of non-aqueous-base drilling fluids has tended to restrict their use in drilling operations to wells drilled in water sensitive formations, complex three dimensional wells, high temperature wells, and, very often, to just the lower portion of such wells.
In the case of non-aqueous-base packer fluids, these wellbore fluids have severe drawbacks when employed to complete deep, high pressure, high temperature wells. This is due to the fact that the requisite packer fluid density increases with increasing well depth and, therefore, packer fluids used in deep wells tend to be quite dense. Accordingly, non-aqueous-base packer fluids used in deep wells contain significant concentrations of one or more weighting agents to achieve the required density. One disadvantage of employing these non-aqueous-base packer fluids is that, due to gravity, the weighting agent eventually settles out on top of the packer, creating a hard, cement-like plug that could be roughly 100 m (328 ft) or more thick. (Furthermore, at downhole temperatures of approximately 148.9.degree. C. (300.degree. F.) and greater, the settling out of the weighting agent is accelerated due to the thermal decomposition of suspending agents present in the non-aqueous-base packer fluids.) Penetrating such plugs makes workover operations extremely costly and difficult.
Conventional non-aqueous-base drilling fluids are also not suitable for use as completion fluids. For example, non-aqueous-base drilling fluids are commonly brine-in-oil emulsions and emulsifiers, which are present in such emulsions, can produce detrimental formation wettability changes. Moreover, the numerous ingredients present in conventional non-aqueous-base drilling fluids cause plugging and other types of damage to the formation. In addition, emulsified brine droplets can plug subterranean formations.